Engine knocking (also referred to as detonation, spark knocking or pinging) occurs in internal combustion engines when a portion of an air-fuel mixture in an engine cylinder does not combust with the rest of the air-fuel mixture in the cylinder at the precise time in a piston's stroke cycle. These create peripheral explosions which can be heard by a user. Knocking can limit the compression ratio, ignition timing, and other engine parameters that effect efficiency of IC engines.
A contributing factor to knocking is engine cylinder temperature. Too high a temperature of the engine cylinder can auto-ignite a portion of the air-fuel mixture before a combustion stroke causing knocking. One way of preventing knocking is to cool the cylinder of the engine to prevent auto ignition of the fuel (e.g., diesel fuel communicated into the engine).
Liquids can be used to cool the engine cylinder. Conventional methods of cooling IC engines include humid air and water injection, which are related processes. Humid air cycles were used in reciprocating engines and gas turbines to reduce NOx emissions. Humid air involves inserting steam or any other non-reactive vapor into the air-fuel stream communicated to the cylinder. Vapor injection increases the thermal mass of the air-fuel mixture and dilutes the charge air. The dilution decreases the tendency of to knock while maintaining stoichiometric combustion. The larger thermal mass reduces the peak temperatures in the combustion chamber therefore reducing NOx formation.
Because humid air is already in a vapor phase when mixed with the air-fuel stream, it cannot go through a phase change during compression. When humid air is compressed, the temperature increases almost as fast as if the air were dry. The temperature rises slightly lower since the specific heat ratio of the vapor e.g., water, is often lower than the specific heat ratio of the air/fuel mixture. Any improved knock margin is primarily dependent upon the dilution effect.
In water injection, large liquid droplets are added to an air-fuel stream to provide cooling from the phase change from liquid to gaseous state. The phase change occurs during each of the compression stroke and the combustion stroke in a cylinder of the piston. Having liquid droplets during the combustion phase can provide cooling but can reduce the efficiency of combustion. Upon insertion, the water droplets can follow one of two paths: collision with internal surfaces of the cylinder or entrainment in the airflow.
Large droplets may collide with internal surfaces of the cylinder during airflow direction changes. The inertia of large water droplets can overwhelm the friction from surrounding airflow and hinder the droplets ability to follow air flow changes around structures within air ducting, intake manifold, and cylinder causing droplet collision with the internal surfaces. If the surface is hot such as in the cylinder, a portion of the droplets may vaporize and cause some cooling by the phase change. Most of the droplets, however, do not vaporize during the compression stroke and are pushed by the airflow along the internal surface. As the liquid moves, it may coalesce with other liquid droplets. During such coalescing, the ratio of surface area to volume of the liquid decreases, which reduces the effects of air temperature on liquid vaporization. Surface wetting by the liquid droplets may also lead to corrosion which can cause cylinder liner scuffing and oil quality degradation.
If the droplets do not collide with a solid surface, they may remain in the airflow. Eventually, at least some of the droplets may arrive in the cylinder. During compression, the air temperature surrounding the droplets increases. Increased temperature causes heat to flow from the air to the water droplet. As the thermal gradient between the droplet and surrounding air increases, the heat transfer rate increases. Heat transfer across the large thermal gradients associated with large droplets increases system entropy reducing cycle efficiency.
Large droplets, for example having a mean diameter of greater than about 20 microns, have enough thermal inertia to remain in liquid form during combustion. Large droplets have a lower surface area to volume ratio than small droplets. Because heat transfer depends upon surface area and heat required for total vaporization depends upon volume, large droplets may not have enough time to completely vaporize. When the large droplets are present during combustion, they lower peak cylinder pressures and temperatures and slow flame propagation which decreases system efficiency. While water injection and humid air injection may have some ability to reduce engine knock, various shortcomings persist including entropy increase from large thermal gradients required during liquid vaporization, corrosion from surface wetting and undesired cooling of the combustion process.